Scientists at Toshiba Research Europe Ltd. and at Cambridge University, both in Cambridge, UK, have demonstrated that single photons may be detected in a quantum dot device by monitoring changes in the resonant tunneling current through a double-barrier structure. Describing the work in the Feb. 18 issue of Physical Review Letters, they report detection efficiencies of 12.5 percent, which they estimate may be improved to approximately 65 percent with optimization of the device structure.The detector is a GaAs NIN diode, with the intrinsic region featuring a double-barrier tunnel structure of AlGaAs and a layer of InAs quantum dots. The researchers selected the growth conditions for the fabrication of the dots to yield an areal density of approximately 1010/cm2. Under a forward bias close to resonance, the detector displays a discrete, upward step in its tunnel current each time that a quantum dot captures a photon, an effect that the investigators observed at operating temperatures of up to 77 K. Tests under 550-nm illumination indicate that the device has a quantum efficiency of approximately 11 percent.Such a single-photon detector may find application in quantum key distribution systems. The effect also may be exploited to produce single-electron detectors for quantum computing, the scientists suggest.